In the case where a semiconductor package consuming a large amount of is electricity such as a Central Processing Unit (CPU) and an optoelectronic module is mounted on a board and the like to be used in a computer, means for dissipating heat from the semiconductor package is generally required.
Japanese laid-open publication No. H09-321188 discloses a structure for mounting a semiconductor package as shown in FIG. 1. The semiconductor package shown in FIG. 1 is mounted on board 51. And heat sink 54 is placed on top of semiconductor package 52 mounted on board 51. Heat sink 54 is in tight contact with the top surface of semiconductor package 52 due to the elastic force of leaf spring 55. The heat sink used in that mounting structure is made of excellent heat conductivity material, and is shaped to efficiently dissipate absorbed heat into the air.
In some cases, however, it is impossible or difficult to mount a heat sink on top of a semiconductor package depending to the shape of the semiconductor package. For example, semiconductor package 60 shown in FIG. 2 is an optoelectronic module, with light input/output part 61 provided on the top portion thereof. This structure makes it impossible to mount the heat sink on top of this semiconductor package. Accordingly, in the example shown in FIG. 2, a structure for dissipating heat from the underside of semiconductor package 60 through solder balls 63 to mounting board 62 is employed. The mounting structure shown in FIG. 2 is disclosed in OPTRONICS (2005), No. 1, page 184.
However, most mounting boards are made of resin and have great thermal resistance. Accordingly, this structure cannot sufficiently dissipate heat from the optoelectronic module that generates a large amount of heat. As a mounting structure for overcoming that problem, the specification of Japanese laid-open publication No. H07-283349 discloses a mounting structure shown in FIG. 3. Specifically, semiconductor package 72 is mounted on the front surface of mounting board 71 in which through-holes 70 are formed, and heat sink 73 is mounted on the rear surface of mounting board 71. In the mounting structure shown in FIG. 3, heat generated from semiconductor package 72 is transferred through through-holes 70 and the like to heat sink 73, which in turn dissipates the heat from the surface into the air. In the mounting structure shown in FIG. 3, since Charge-Coupled Device (CCD) image sensor is mounted on the top surface of the semiconductor package 72, the heat from semiconductor package 72 is dissipated through the opposite surface.